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<div class="document" id="resource-arbitration">
<h1 class="title">Resource Arbitration</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr class="field"><th class="docinfo-name">TEP:</th><td class="field-body">108</td>
</tr>
<tr class="field"><th class="docinfo-name">Group:</th><td class="field-body">Core Working Group</td>
</tr>
<tr class="field"><th class="docinfo-name">Type:</th><td class="field-body">Documentary</td>
</tr>
<tr><th class="docinfo-name">Status:</th>
<td>Final</td></tr>
<tr class="field"><th class="docinfo-name">TinyOS-Version:</th><td class="field-body">2.x</td>
</tr>
<tr><th class="docinfo-name">Authors:</th>
<td>Kevin Klues
<br />Philip Levis
<br />David Gay
<br />David Culler
<br />Vlado Handziski</td></tr>
</tbody>
</table>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">This memo documents a part of TinyOS for the TinyOS Community, and
requests discussion and suggestions for improvements.  Distribution
of this memo is unlimited. This memo is in full compliance with
TEP 1.</p>
</div>
<div class="section">
<h1><a id="abstract" name="abstract">Abstract</a></h1>
<p>This memo documents the general resource sharing mechanisms for TinyOS
2.x. These mechanisms are used to allow multiple software components to
arbitrate access to shared abstractions.</p>
</div>
<div class="section">
<h1><a id="introduction" name="introduction">1. Introduction</a></h1>
<p>TinyOS 1.x has two mechanisms for managing shared resources:
virtualization and completion events. A virtualized resource appears
as an independent instance of an abstraction, such as the Timer
interface in TimerC. A client of a Timer instance can use it
independently of the others: TimerC virtualizes the underlying
hardware clock into N separate timers.</p>
<p>Some abstractions, however, are not well suited to virtualization:
programs need the control provided by a physical abstraction. For
example, components in 1.x share a single communication stack,
GenericComm.  GenericComm can only handle one outgoing packet at a
time. If a component tries to send a packet when GenericComm is
already busy, then the call returns FAIL. The component needs a way to
tell when GenericComm is free so it can retry. TinyOS
1.x provides the mechanism of a global completion event which is
signaled whenever a packet send completes. Interested components can
handle this event and retry.</p>
<p>This approach to physical (rather than virtualized) abstractions
has several drawbacks:</p>
<ul class="simple">
<li>If you need to make several requests, you have to handle the
possibility of a request returning FAIL at any point. This complicates
implementations by adding internal states.</li>
<li>You have no control over the timing of a sequence of operations. One
example of when this can be a problem is timing-sensitive use of an
A/D converter.  You need a way to pre-reserve the use of the ADC so
that its operations can be run at the exact moment they are desired.</li>
<li>If a hardware resource supports reservation, you cannot express this
via this software interface. For instance, I2C buses have a
concept of &quot;repeated start&quot; when doing multiple bus transactions,
but it is not clear how to use this in TinyOS 1.x's I2C abstraction.</li>
<li>Most TinyOS 1.x services do not provide a very convenient way of
monitoring an abstraction's availability for the purpose of retries,
nor very clear documentation of which requests could happen simultaneously.</li>
</ul>
<p>It should be clear that a single approach to resource sharing is not appropriate
for all circumstances. For instance, requiring explicit reservation of a
resource allows programs to have better timing guarantees for access to an A/D
converter. If a program does not need precise timing guarantees, however (e.g.,
when measuring temperature in a biological monitoring application), this extra
resource reservation step unnecessarily complicates code and can be handled
nicely using virtualization. The following section introduces the concept of
resource classes in order to address this issue.  The sharing policy used by a
particular resource abstraction is dictated by the resource class it belongs to.</p>
</div>
<div class="section">
<h1><a id="resource-classes" name="resource-classes">2. Resource Classes</a></h1>
<p>TinyOS 2.x distinguishes between three kinds of abstractions:
<em>dedicated</em>, <em>virtualized</em>, and <em>shared</em>.  Components offer resource
sharing mechanisms appropriate to their goals and level of abstraction.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">It is important to point out that Hardware Presentation Layer (HPL)
components of the HAA <a class="footnote-reference" href="#id4" id="id1" name="id1">[1]</a> can never be virtualized, as virtualization
inevitably requires state. Depending on their
expected use, HPL abstractions can either be dedicated or
shared. For example, while hardware timers are rarely
multiplexed between multiple components, buses almost always are.
This can be seen on the MSP430 microcontroller, where the compare and
counter registers are implemented as dedicated resources, and the USARTs
are shared ones.</p>
</div>
<div class="section">
<h2><a id="dedicated" name="dedicated">2.1 Dedicated</a></h2>
<p>An abstraction is <em>dedicated</em> if it is a resource
which a subsystem needs exclusive access to at all times.
In this class of resources, no sharing policy is needed since only
a single component ever requires use of the resource.  Examples of
dedicated abstractions include interrupts and counters.</p>
<p>Dedicated abstractions MAY be annotated with the nesC attribute
&#64;atmostonce or &#64;exactlyonce to provide compile-time checks that
their usage assumptions are not violated.</p>
<p>Please refer to Appendix A for an example of how a dedicated
resource might be represented, including the use of
the nesC &#64;exactlyonce attribute.</p>
</div>
<div class="section">
<h2><a id="virtualized" name="virtualized">2.2 Virtualized</a></h2>
<p><em>Virtual</em> abstractions hide multiple clients from each other
through software virtualization. Every client of a virtualized resource
interacts with it as if it were a dedicated resource, with all virtualized
instances being multiplexed on top of a single underlying resource. Because
the virtualization is done in software, there is no upper bound on the number
of clients using the abstraction, barring memory or efficiency constraints.
As virtualization usually requires keeping state that scales with the number
of virtualized instances, virtualized resources often use the Service Instance
pattern <a class="footnote-reference" href="#id6" id="id2" name="id2">[3]</a>, which is based on a parameterized interface.</p>
<p>Virtualization generally provides a very simple interface to its clients.
This simplicity comes at the cost of reduced efficiency and an inability to
precisely control the underlying resource. For example, a virtualized
timer resource introduces CPU overhead from dispatching and maintaining
each individual virtual timer, as well as introducing jitter whenever two
timers happen to fire at the same time. Please refer to Appendix A for an
example of how such a virtualized timer resource might be implemented.</p>
</div>
<div class="section">
<h2><a id="shared" name="shared">2.3 Shared</a></h2>
<p>Dedicated abstractions are useful when a resource is
always controlled by a single component. Virtualized abstractions are
useful when clients are willing to pay a bit of overhead and sacrifice
control in order to share a resource in a simple way. There are
situations, however, when many clients need precise control of a
resource. Clearly, they can't all have such control at the same time:
some degree of multiplexing is needed.</p>
<p>A motivating example of a shared resource is a bus.
The bus may have multiple peripherals on it, corresponding to
different subsystems. For example, on the Telos platform the flash
chip (storage) and the radio (network) share a bus. The storage and
network stacks need exclusive access to the bus when using it,
but they also need to share it with the other subsystem. In this
case, virtualization is problematic, as the radio stack needs to be
able to perform a series of operations in quick succession without
having to reacquire the bus in each case. Having the bus be a
shared resource allows the radio stack to send a series of operations
to the radio atomically, without having to buffer them all up
in memory beforehand (introducing memory pressure in the process).</p>
<p>In TinyOS 2.x, a resource <em>arbiter</em> is responsible for multiplexing
between the different clients of a shared resource. It determines
which client has access to the resource at which time. While a client
holds a resource, it has complete and unfettered control. Arbiters assume
that clients are cooperative, only acquiring the resource when needed
and holding on to it no longer than necessary. Clients explicitly
release resources: there is no way for an arbiter to forcibly reclaim it.
The following section is dedicated to describing the arbiter and its
interfaces.</p>
</div>
</div>
<div class="section">
<h1><a id="resource-arbiters" name="resource-arbiters">3. Resource Arbiters</a></h1>
<p>Every shared resource has an arbiter to manage which client
can use the resource at any given time. Because an arbiter is a
centralized place that knows whether the resource is in use, it can also
provide information useful for a variety of other services, such as
power management. An arbiter MUST provide a parameterized Resource
interface as well as an instance of the ArbiterInfo interface. The Resource
interface is instantiated by different clients wanting to gain access to a
resource.  The ArbiterInfo interface is used by components that wish to
retrieve global information about the status of a resource (i.e. if it is in
use, who is using it, etc.).  An arbiter SHOULD also provide a parameterized
ResourceRequested interface and use a parameterized ResourceConfigure interface.
It MAY also provide an instance of the ResourceDefaultOwner interface or
any additional interfaces specific to the particular arbitration policy
being implemented.  Each of these interfaces is explained in greater detail below:</p>
<pre class="literal-block">
Resource     ArbiterInfo ResourceRequested     ResourceDefaultOwner
   |                |         |                        |
   |                |         |                        |
   |               \|/       \|/                       |
   |             \---------------/                     |
   |--------------|   Arbiter   |----------------------|
                 /---------------\
                         |
                         |
                        \|/
                  ResourceConfigure
</pre>
<div class="section">
<h2><a id="resource" name="resource">3.1 Resource</a></h2>
<p>Clients of an arbiter request access
to a shared resource using the Resource interface:</p>
<pre class="literal-block">
interface Resource {
  async command error_t request();
  async command error_t immediateRequest();
  event void granted();
  async command error_t release();
  async command bool isOwner();
}
</pre>
<p>A client lets an arbiter know it needs access to a resource by
making a call to request(). If the resource is free,
SUCCESS is returned, and a granted event is signaled
back to the client.  If the resource is busy, SUCCESS will
still be returned, but the request will be queued
according to the queuing policy of the arbiter. Whenever a client is
done with the resource, it calls the release() command, and the next
client in the request queue is given access to the resource and
is signaled its granted() event. If a client ever makes multiple
requests before receiving a granted event, an EBUSY value is returned,
and the request is not queued.  Using this policy, clients are not able to
monolopize the resource queue by making multiple requests, but they may still be
able to monopolize the use of the resource if they do not release it in a
timely manner.</p>
<p>Clients can also request the use of a resource through the
immediateRequest() command.  A call to immediateRequest() can either
return SUCCESS or FAIL, with requests made through this command never being
queued.  If a call to immediateRequest() returns SUCCESS, the client is granted
access to the resource immediately after the call has returned, and no granted
event is ever signaled.  If it returns FAIL, the client is not granted access to
the resource and the request does not get queued.  The client will have to try
and gain access to the resource again later.</p>
<p>A client can use the isOwner command of the Resource interface to
check if it is the current owner of the resource.  This command is mostly
used to perform runtime checks to make sure that clients not owning a resource
are not able to use it.  If a call to isOwner fails, then no call
should be made to commands provided by that resource.</p>
<p>The diagram below shows how a simple shared resource can be
built from a dedicated resource by using just the Resource interface
provided by an arbiter.:</p>
<pre class="literal-block">
         /|\                    /|\
          |                      |
          | Data Interface       | Resource
          |                      |
--------------------------------------------
|               Shared Resource            |
--------------------------------------------
         /|\                    /|\
          |                      |
          | Data Interface       | Resource
          |                      |
----------------------      ----------------
| Dedicated Resource |      |    Arbiter   |
----------------------      ----------------
</pre>
<p>An arbiter MUST provide exactly one parameterized Resource interface,
where the parameter is a client ID, following the Service
Instance pattern[3]_.  An arbitrated component SomeNameP MUST
#define SOME_NAME_RESOURCE to a string which can be passed to unique()
to obtain a client id.  This #define must be placed in a separate file
because of the way nesC files are preprocessed: including the
SomeNameP component isn't enough to ensure that macros #define'd in
SomeNameP are visible in the referring component.</p>
<p>Please refer to Appendix B for an example of how to wrap a component of this type
inside a generic configuration. Wrapping the component in this way ensures that
each Resource client is given a unique client ID, with the added
benefit of properly coupling multiple components that all need to
refer to the same client ID.</p>
<p>Appendix B also provides a complete example of how an I2C resource might be
abstracted according to this pattern.  For further examples see the various
chip implementations in the tinyos-2.x source tree under tinyos-2.x/chips/</p>
</div>
<div class="section">
<h2><a id="arbiterinfo" name="arbiterinfo">3.2 ArbiterInfo</a></h2>
<p>Arbiters MUST provide an instance of the ArbiterInfo interface.
The ArbiterInfo interface allows a component to query the current
status of an arbiter:</p>
<pre class="literal-block">
interface ArbiterInfo {
  async command bool inUse();
  async command uint8_t clientId();
}
</pre>
<p>In contrast to the parameterized Resource interface provided by an arbiter,
only a single ArbiterInfo interface is provided.  Its purpose is
to allow one to find out:</p>
<ul class="simple">
<li>Whether the resource for which it is arbitrating use is currently in use or not</li>
<li>Which client is using it.</li>
</ul>
<p>One can view ArbiterInfo as an interface for obtaining global information about
the use of a resource, while Resource can be viewed as an interface for obtaining
local access to that resource.</p>
<p>The primary use of the ArbiterInfo interface is to allow a shared resource to reject
any calls made through its data interface by clients that do not currently have access to
it.  For an example of how this interface is used in this fashion refer to Appendix B.:</p>
<pre class="literal-block">
         /|\                        /|\
          |                          |
          | Data Interface           | Resource
          |                          |
-----------------------------------------------------------
|                     Shared Resource                     |
-----------------------------------------------------------
         /|\                    /|\         /|\
          |                      |           |
          | Data Interface       | Resource  | ArbiterInfo
          |                      |           |
----------------------      -------------------------------
| Dedicated Resource |      |           Arbiter           |
----------------------      -------------------------------
</pre>
</div>
<div class="section">
<h2><a id="resourcerequested" name="resourcerequested">3.3 ResourceRequested</a></h2>
<p>Sometimes it is useful for a client to be able to hold onto a resource until
someone else needs it and only at that time decide to release it.  Using the
ResourceRequested interface, this information is made available to the current
owner of a resource:</p>
<pre class="literal-block">
interface ResourceRequested {
  async event void requested();
  async event void immediateRequested();
}
</pre>
<p>A requested event is signaled to the current owner of the resource if another
client makes a request for the resource through the request() command of
its Resource interface.  If a request is made through the immediateRequest()
command, then the immediateRequested() event is signaled.</p>
<p>An arbiter SHOULD provide a parameterized ResourceRequested interface to its
clients, but is not required to.  The client id of the parameterized
ResourceRequested interface should be coupled with the client id of the Resource
interface to ensure that all events are signaled to the proper clients.  Please
refer to Appendix B for an example of how this interface might be used.:</p>
<pre class="literal-block">
         /|\                        /|\                   /|\
          |                          |                     |
          | Data Interface           | Resource            | ResourceRequested
          |                          |                     |
--------------------------------------------------------------------------------
|                              Shared Resource                                 |
--------------------------------------------------------------------------------
         /|\                    /|\         /|\            /|\
          |                      |           |              |
          | Data Interface       | Resource  | ArbiterInfo  | ResourceRequested
          |                      |           |              |
----------------------      ----------------------------------------------------
| Dedicated Resource |      |                     Arbiter                      |
----------------------      ----------------------------------------------------
</pre>
</div>
<div class="section">
<h2><a id="resourceconfigure" name="resourceconfigure">3.4 ResourceConfigure</a></h2>
<p>The existence of the ResourceConfigure interface allows a resource to be
automatically configured just before a client is granted access to it.
Components providing the ResourceConfigure interface use the interfaces
provided by an underlying dedicated resource to configure it into one
of its desired modes of operation.  A cleint then wires its shared resource
abstraction to the component implementing the desired configuration.  The
configure command is called immediataely before the client is granted access
to the resource, and the unconfigure command is called just before fully
releasing it.:</p>
<pre class="literal-block">
interface ResourceConfigure {
  async command void configure();
  async command void unconfigure();
}
</pre>
<pre class="literal-block">
  ResourceConfigure       ResourceConfigure      ResourceConfigure
         |                       |                     /|\
         |                       |                      |
        \|/                     \|/                     |
-------------------     -------------------    -------------------
| Configuration 1 |     | Configuration 2 |    | Shared Resource |
-------------------     -------------------    -------------------
         |                       |                     /|\
         |   Control Interface   |                      | ResourceConfigure
        \|/                     \|/                     |
      ------------------------------               -----------
      |     Dedicated Resource     |               | Arbiter |
      ------------------------------               -----------
</pre>
<p>The arbiter SHOULD use a parameterized ResourceConfigure interface, with
its client ID parameter coupled with the client id of its parameterized
Resource interface.  If an arbiter uses the ResourceConfigure interface,
it MUST call ResourceConfigure.configure() on the granted client ID
before it signals the Resource.granted() event. Similarly, after a valid
call to Resource.release(), it MUST call ResourceConfigure.unconfigure()
on the releasing client ID.  By calling ResourceConfigure.configure() just
before granting a client access to a resource and calling
ResourceConfigure.unconfigure() just before fully releasing it, it is guaranteed
that a resource is always unconfigured before an attempt to configure it can be
made again.</p>
<p>The commands included in this interface could have been made part of the standard
Resource interface (and changed into callback events), but at a higher cost than
keeping them separate. Introducing these new commands into the Resource interface
would have lead to a large number of clients all including redundant configuration
code, while using the call out approach to a separate component ensures that we
only have a single instance of the code.</p>
<p>For an example of how configurations for the three different modes of the
Msp430 Usart component can take advantage of the ResourceConfigure
interface refer to Appendix B as well as section 4 on the use of
cross-component reservation.</p>
</div>
<div class="section">
<h2><a id="resourcedefaultowner" name="resourcedefaultowner">3.5 ResourceDefaultOwner</a></h2>
<p>The normal Resource interface is for use by clients that all share the resource
in an equal fashion. The ResourceDefaultOwner interface is for use by a single
client that needs to be given control of the resource whenever no one else is
using it. An arbiter MAY provide a single instance of the ResourceDefaultOwner
interface. It MUST NOT provide more than one.:</p>
<pre class="literal-block">
interface ResourceDefaultOwner {
  async event void granted();
  async command error_t release();
  async command bool isOwner();
  async event void requested();
  async event void immediateRequested();
}
</pre>
<p>The Arbiter MUST guarantee that the client of the ResourceDefaulrClient interface is
made the owner of the resource before the boot initialization sequence is
completed.  When a normal resource client makes a request for the resource, the
ResourceDefaultOwner will receive either a requested() or an immediateRequested()
event depending on how the request was made.  It must then decide if and when to
release it.  Once released, all clients that have pending requests will be
granted access to the resource in the order determined by the queuing policy of
the arbiter in use.  Once all pending requests have been granted (including
those that came in while other clients had access to the resource), the
ResourceDefaultOwner is automatically given control of the resource, receiving its
granted() event in the process.  The ResourceDefaultOwner interface also contains
the same isOwner() command as the normal Resource interface, and the semantics
of its use are exactly the same.</p>
<p>Although the ResourceDefaultOwner interface looks similar to a combination of the
normal Resource interface and the ResourceRequested interface, its intended use
is quite different.  The ResourceDefaultOwner interface should only be used by
clients that wish to have access to a resource only when no other clients are
using it.  They do not actively seek access to the resource, but rather use
it to perform operations when it would otherwise simply be idle.</p>
<p>The primary motivation behind the definition of the ResourceDefaultOwner
interface is to allow for an easy integration of power management
for a resource with its arbitration policy.  Arbiters that want to allow
a resource to be controlled by a particular power management policy can
provide the ResourceDefaultOwner interface for use by a component that
implements that policy. The power management component will receive the
granted() event whenever the resource has gone idle, and will proceed in
powering it down.  When another client requests the resource, the power
manager will be notified through either the requested() or
immediateRequested() events as appropriate.  It can then power up the resource
and release it once the power up has completed.  Note that if power up is
a split-phase operation (takes a while), then calls by clients to
immediateRequest() when in the powered down state will return
FAIL. Please see the TEP on the Power Management of Non-Virtualized devices
(<a class="footnote-reference" href="#id7" id="id3" name="id3">[4]</a>) for more details.</p>
</div>
</div>
<div class="section">
<h1><a id="cross-component-reservation" name="cross-component-reservation">4. Cross-Component Reservation</a></h1>
<p>In some cases, it is desirable to share the reservation of a
single resource across multiple components. For example, on the TI
MSP430, a single USART component can be used as an I2C bus, a UART,
or an SPI connection. Clearly, on this chip, a reservation of the I2C
bus implicitly restricts the corresponding UART and SPI
services from gaining access to the resource. Enforcing such a policy
can be accomplished in the framework described above by:</p>
<blockquote>
<ol class="arabic simple">
<li>Creating a set of unique ids for each service using the shared
resource.</li>
<li>Mapping these ids onto the ids of the underlying resource</li>
</ol>
</blockquote>
<p>Clients connecting to these services do not know that that this
mapping is taking place.  As far as they are concerned, the only
arbitration taking place is between other clients using the same
service.  In the MSP430 example, a single client of the I2C bus could
be contending with a single client of the SPI connection, but they
would probably have the same service level client ID.  These two
service level client ids would be mapped onto 2 unique resource ids
for use by the shared USART component. The proper way to achieve this
mapping is through the use of generic components.  The example given
below shows how to perform this mapping for the SPI component on the
MSP430.  It is done similarly for the UART and I2C bus:</p>
<pre class="literal-block">
#include &quot;Msp430Usart.h&quot;
generic configuration Msp430Spi0C() {
  provides interface Resource;
  provides interface SpiByte;
  provides interface SpiPacket;
}
implementation {
  enum { CLIENT_ID = unique(MSP430_SPIO_BUS) };

  components Msp430Spi0P as SpiP;
  Resource = SpiP.Resource[ CLIENT_ID ];
  SpiByte = SpiP.SpiByte;
  SpiPacket = SpiP.SpiPacket[ CLIENT_ID ];

  components new Msp430Usart0C() as UsartC;
  SpiP.UsartResource[ CLIENT_ID ] -&gt; UsartC.Resource;
  SpiP.UsartInterrupts -&gt; UsartC.HplMsp430UsartInterrupts;
}
</pre>
<p>The definition of the MSP430_SPIO_BUS string is defined in
Msp430Usart.h. A unique id is created from this string every time a
new Msp430Spi0C component is instantiated.  This id is used as a
parameter to the parameterized Resource interface provided by the
Msp430Spi0P component.  This is where the mapping of the two
different ids begins.  As well as <em>providing</em> a parameterized
Resource interface (Msp430Spi0P.Resource), the Msp430Spi0P component
also <em>uses</em> a parameterized Resource interface (Msp430Spi0P.UsartResource).
Whenever a client makes a call through the provided Resource interface
with id CLIENT_ID, an underlying call to the Msp430Spi0P.Resource interface
with the same id is implicitly made.  By then wiring the Msp430Spi0P.UsartResource
interface with id CLIENT_ID to an instance of the Resource interface
provided by the instantiation of the Msp430Usart0C component, the mapping
is complete. Any calls to the Resource interface provided by a new
instantiation of the Msp430Spi0C component will now be made through a
unique Resource interface on the underlying  Msp430Usart0C component.</p>
<p>This level of indirection is necessary because it may not always be
desirable to directly wire the service level Resource interface to
the underlying shared Resource interface.  Sometimes we may want to
perform some operations between a service level command being
called, and calling the underlying command on the shared resource.
With such a mapping, inserting these operations is made possible.</p>
<p>Having such a mapping is also important for services that need to
explicitly keep track of the number of clients they have,
independent from how many total clients the underlying shared
resource has.  For example, a sensor implementation that uses an
underlying ADC resource may wish to power down its sensor whenever it
has no clients.  It doesn't want to have to wait until the entire ADC
is free to do so.  Providing this mapping allows the implicit power
manager components described in TEP 115 to be wired in at both levels
of the abstraction without interfering with one another.  In this
way, implementations of these components become much simpler, and code
reuse is encouraged.</p>
<p>Implementations of components similar to this one can be found in the
tinyos-2.x source tree in the tos/chips/msp430/uart directory</p>
</div>
<div class="section">
<h1><a id="implementation" name="implementation">5. Implementation</a></h1>
<p>Because most components use one of a small number of arbitration
policies, tinyos-2.x includes a number of default resource arbiters. These
arbiters can be found in <tt class="docutils literal"><span class="pre">tinyos-2.x/tos/system</span></tt> and are all
generic components that include one of the two signatures seen below:</p>
<pre class="literal-block">
generic module SimpleArbiter {
  provides interface Resource[uint8_t id];
  provides interface ResourceRequested[uint8_t id];
  provides interface ArbiterInfo;
  uses interface ResourceConfigure[uint8_t id];
}

generic module Arbiter {
  provides interface Resource[uint8_t id];
  provides interface ResourceRequested[uint8_t id];
  provides interface ResourceDefaultOwner;
  provides interface ArbiterInfo;
  uses interface ResourceConfigure[uint8_t id];
}
</pre>
<p>The &quot;Simple&quot; arbiters are intended for use by resources that
do not require the additional overhead incurred by providing the
ResourceDefaultOwner interface.</p>
<p>For many situations, changing an arbitration policy requires nothing
more than changing the queuing policy it uses to decide the order in
which incoming requests should be granted. In this way, separating
queuing policy implementations from actual arbitration implementations
encourages code reuse.  The introduction of the SimpleArbiterP and
ArbiterP components found under tinyos-2.x/tos/system help in this
separation.  They can be wired to components providing
a particular queuing policy through the use of the ResourceQueue
interface.:</p>
<pre class="literal-block">
interface ResourceQueue {
  async command bool isEmpty();
  async command bool isEnqueued(resource_client_id_t id);
  async command resource_client_id_t dequeue();
  async command error_t enqueue(resource_client_id_t id);
}
</pre>
<p>An example of wiring a First-Come-First-Serve (FCFS) queuing policy to
the SimpleArbiterP component using the ResourceQueue interface
defined above can be seen below:</p>
<pre class="literal-block">
generic configuration SimpleFcfsArbiterC(char resourceName[]) {
  provides {
    interface Resource[uint8_t id];
    interface ResourceRequested[uint8_t id];
    interface ArbiterInfo;
  }
  uses interface ResourceConfigure[uint8_t id];
}
implementation {
  components MainC;
  components new FcfsResourceQueueC(uniqueCount(resourceName)) as Queue;
  components new SimpleArbiterP() as Arbiter;

  MainC.SoftwareInit -&gt; Queue;

  Resource = Arbiter;
  ResourceRequested = Arbiter;
  ArbiterInfo = Arbiter;
  ResourceConfigure = Arbiter;

  Arbiter.Queue -&gt; Queue;
}
</pre>
<p>This generic configuration can be instantiated by a resource in order
to grant requests made by its clients in an FCFS fashion.</p>
<p>All of the default queuing policies provided in tinyos-2.x along with the
respective arbitration components that have been built using them are
given below:</p>
<p>Queuing Policies:</p>
<ul class="simple">
<li>FcfsResourceQueueC</li>
<li>RoundRobinResourceQueueC</li>
</ul>
<p>Arbiters:</p>
<ul class="simple">
<li>SimpleFcfsArbiterC</li>
<li>FcfsArbiterC</li>
<li>SimpleRoundRobinArbiterC</li>
<li>RoundRobinArbiterC</li>
</ul>
<p>Keep in mind that neither the implementation of an arbiter nor its
queuing policy can be used to explicitly restrict access to an
underlying shared resource.  The arbiter simply provides a standardized
way of managing client ids so that shared resources don't have to duplicate
this functionality themselves every time they are implemented.  In order to
actually restrict clients from using a resource without first requesting it,
a shared resource must use the functionality provided by the ArbiterInfo interface
to perform runtime checks on the current owner of a resource.  Please refer
to the section on the ArbiterInfo interface in Appendix B for more information
on how such runtime checks can be performed.</p>
</div>
<div class="section">
<h1><a id="author-s-address" name="author-s-address">6. Author's Address</a></h1>
<div class="line-block">
<div class="line">Kevin Klues</div>
<div class="line">503 Bryan Hall</div>
<div class="line">Washington University</div>
<div class="line">St. Louis, MO 63130</div>
<div class="line"><br /></div>
<div class="line">phone - +1-314-935-6355</div>
<div class="line">email - <a class="reference" href="mailto:klueska&#64;cs.wustl.edu">klueska&#64;cs.wustl.edu</a></div>
<div class="line"><br /></div>
<div class="line">Philip Levis</div>
<div class="line">358 Gates Hall</div>
<div class="line">Stanford University</div>
<div class="line">Stanford, CA 94305-9030</div>
<div class="line"><br /></div>
<div class="line">phone - +1 650 725 9046</div>
<div class="line">email - <a class="reference" href="mailto:pal&#64;cs.stanford.edu">pal&#64;cs.stanford.edu</a></div>
<div class="line"><br /></div>
<div class="line">David Gay</div>
<div class="line">2150 Shattuck Ave, Suite 1300</div>
<div class="line">Intel Research</div>
<div class="line">Berkeley, CA 94704</div>
<div class="line"><br /></div>
<div class="line">phone - +1 510 495 3055</div>
<div class="line">email - <a class="reference" href="mailto:david.e.gay&#64;intel.com">david.e.gay&#64;intel.com</a></div>
<div class="line"><br /></div>
<div class="line">David Culler</div>
<div class="line">627 Soda Hall</div>
<div class="line">UC Berkeley</div>
<div class="line">Berkeley, CA 94720</div>
<div class="line"><br /></div>
<div class="line">phone - +1 510 643 7572</div>
<div class="line">email - <a class="reference" href="mailto:culler&#64;cs.berkeley.edu">culler&#64;cs.berkeley.edu</a></div>
<div class="line"><br /></div>
<div class="line">Vlado Handziski</div>
<div class="line">Sekr FT5</div>
<div class="line">Einsteinufer 25</div>
<div class="line">10587 Berlin</div>
<div class="line">GERMANY</div>
<div class="line"><br /></div>
<div class="line">email - <a class="reference" href="mailto:handzisk&#64;tkn.tu-berlin.de">handzisk&#64;tkn.tu-berlin.de</a></div>
</div>
</div>
<div class="section">
<h1><a id="citations" name="citations">7. Citations</a></h1>
<table class="docutils footnote" frame="void" id="id4" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1" name="id4">[1]</a></td><td>TEP 2: Hardware Abstraction Architecture.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id5" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id10" name="id5">[2]</a></td><td>TEP 102: Timers.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id6" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2" name="id6">[3]</a></td><td>Service Instance Pattern. In <em>Software Design Patterns for TinyOS.</em> David Gay, Philip Levis, and David Culler. Published in Proceedings of the ACM SIGPLAN/SIGBED 2005 Conference on Languages, Compilers, and Tools for Embedded Systems (LCTES'05).</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id7" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3" name="id7">[4]</a></td><td>TEP 115: Power Management of Non-Virtualized Devices.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id8" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id9" name="id8">[5]</a></td><td>TinyOS Programming. <a class="reference" href="http://csl.stanford.edu/~pal/pubs/tinyos-programming-1-0.pdf">http://csl.stanford.edu/~pal/pubs/tinyos-programming-1-0.pdf</a></td></tr>
</tbody>
</table>
</div>
<div class="section">
<h1><a id="appendix-a-resource-class-examples" name="appendix-a-resource-class-examples">Appendix A: Resource Class Examples</a></h1>
<div class="section">
<h2><a id="dedicated-resource" name="dedicated-resource">Dedicated Resource</a></h2>
<p>Timer 2 on the Atmega128 microprocessor is a dedicated resource
represented by the HplAtm128Timer2C component:</p>
<pre class="literal-block">
module HplAtm128Timer2C {
  provides {
    interface HplTimer&lt;uint8_t&gt;   as Timer2     &#64;exactlyonce();
    interface HplTimerCtrl8       as Timer2Ctrl &#64;exactlyonce();
    interface HplCompare&lt;uint8_t&gt; as Compare2   &#64;exactlyonce();
  }
}
</pre>
<p>Only a single client can wire to any of these interfaces as enforced through
the nesC &#64;exactlyonce attribute.  Keep in mind that although the interfaces of
this component are only allowed to be wired to once, nothing prevents the
component wiring to them from virtualizing the services they provide at some
higher level.  If you are unfamiliar with how &#64;exactlyonce and other nesC
attributes are used to by the nesC compiler, please refer to section 9.1 of the
TinyOS Programming Manual <a class="footnote-reference" href="#id8" id="id9" name="id9">[5]</a>.</p>
</div>
<div class="section">
<h2><a id="virtualized-resource" name="virtualized-resource">Virtualized Resource</a></h2>
<p>The TimerMilliC component provides a virtual abstraction of millisecond
precision timers to application components <a class="footnote-reference" href="#id5" id="id10" name="id10">[2]</a>. It encapsulates the required
parameterized Timer interface through the use of a generic configuration.
Clients wishing to use a millisecond timer need only instantiate a single
instance of the TimerMilliC generic, leaving the fact that it is virtualized
underneath transparent.:</p>
<pre class="literal-block">
generic configuration TimerMilliC {
  provides interface Timer&lt;TMilli&gt;;
}
implementation {
  components TimerMilliP;
  Timer = TimerMilliP.TimerMilli[unique(UQ_TIMER_MILLI)];
}
</pre>
<p>The actual parameterized Timer interface is provided by the chip specific
HilTimerMilliC component.  This interface is exposed through
the TimerMilliP component which wires HilTimerMilliC to the boot
initialization sequence:</p>
<pre class="literal-block">
configuration TimerMilliP {
  provides interface Timer&lt;TMilli&gt; as TimerMilli[uint8_t num];
}
implementation {
  components HilTimerMilliC, MainC;
  MainC.SoftwareInit -&gt; HilTimerMilliC;
  TimerMilli = HilTimerMilliC;
}
</pre>
</div>
</div>
<div class="section">
<h1><a id="appendix-b-arbiter-interface-examples" name="appendix-b-arbiter-interface-examples">Appendix B: Arbiter Interface Examples</a></h1>
<!-- Note:
Most of the examples provided in this section use complex nesC syntax that may
be unfamiliar to the novice nesC programmer.  Please refer to the TinyOS
programming Manual [5]_ for clarification as necessary. -->
<div class="section">
<h2><a id="id11" name="id11">Resource</a></h2>
<p>Examples of how to use the Resource interface for arbitrating
between multiple clients can be found in the tinyos-2.x
source tree under tinyos-2.x/apps/tests/TestArbiter.</p>
<p>A specific example of where the Resource.isOwner() is used
can be seen in the HplTda5250DataP component of the Infineon
Tda5250 radio implementation:</p>
<pre class="literal-block">
async command error_t HplTda5250Data.tx(uint8_t data) {
  if(call UartResource.isOwner() == FALSE)
    return FAIL;
  call Usart.tx(data);
  return SUCCESS;
}
</pre>
<p>A call to the HplTda5250Data.tx command will fail if the radio does
not currently have control of the underlying Usart resource.  If it
does, then the Usart.tx(data) command is called as requested.</p>
<p>A component using the Resource interface to implement an I2C
service might look like this:</p>
<pre class="literal-block">
#include I2CPacket.h
configuration I2CPacketP {
  provides interface Resource[uint8_t client];
  provides interface I2CPacket&lt;I2CAddrSize&gt;[uint8_t client];
}
implementation {
  components new FcfsArbiterC(I2CPACKET_RESOURCE) as Arbiter;
  components I2CPacketImplP() as I2C;
  ...

  Resource  = Arbiter;
  I2CPacket = I2C;
  ...
}
</pre>
<p>where I2CPacketImplP contains the actual implementation of the
I2C service, and I2CPacket.h contains the #define for the
name of the resource required by the arbiter:</p>
<pre class="literal-block">
#ifndef I2CPACKETC_H
#define I2CPACKETC_H
#define I2CPACKET_RESOURCE &quot;I2CPacket.Resource&quot;
#endif
</pre>
<p>This service would then be made available to a client through
the generic configuration seen below:</p>
<pre class="literal-block">
#include I2CPacket.h
generic configuration I2CPacketC {
  provides interface Resource;
  provides interface I2CPacket&lt;I2CAddrSize&gt;;
}
implementation {
  enum { CLIENT_ID = unique(I2CPACKET_RESOURCE) };

  components I2CPacketP as I2C;
  Resource = I2C.Resource[CLIENT_ID];
  I2CPacket = I2C.I2CPacket[CLIENT_ID];
}
</pre>
<p>In this example, an instance of the I2CPacket interface is coupled with
an instance of the Resource interface on every new instantiation of
the I2CPacketC component.  In this way, a single Resource and a
single I2CPacket interface can be exported by this component together
for use by a client.</p>
<p>Clients of the I2C service would use it as follows:</p>
<pre class="literal-block">
module I2CClientP {
  uses interface Resource as I2CResource;
  uses interface I2CPacket&lt;I2CAddrSize&gt;;
} ...

configuration I2CClientC { }
implementation {
  components I2CClientP, new I2CPacketC();

  I2CClientP.I2CResource -&gt; I2CPacketC.Resource;
  I2CUserM.I2CPacket -&gt; I2CPacket.I2CPacket;
}
</pre>
</div>
<div class="section">
<h2><a id="id12" name="id12">ArbiterInfo</a></h2>
<p>In the implementation of the ADC component on the Msp430 microcontroller,
a simple arbiter is used to provide a round robin sharing policy
between clients of the ADC:</p>
<pre class="literal-block">
configuration Msp430Adc12C {
  provides interface Resource[uint8_t id];
  provides interface Msp430Adc12SingleChannel[uint8_t id];
  provides interface Msp430Adc12FastSingleChannel[uint8_t id];
}
implementation {
  components Msp430Adc12P,MainC,
             new SimpleRoundRobinArbiterC(MSP430ADC12_RESOURCE) as Arbiter,
             ...

  Resource = Arbiter;
  SingleChannel = Msp430Adc12P.SingleChannel;
  FastSingleChannel = Msp430Adc12P.FastSingleChannel;

  Msp430Adc12P.Init &lt;- MainC;
  Msp430Adc12P.ADCArbiterInfo -&gt; Arbiter;
  ...
}
</pre>
<p>In this configuration we see that the Resource interface provided by
Msp430Adc12C is wired directly to the instance of the SimpleRoundRobinArbiterC
component that is created.  The ArbiterInfo interface provided by
SimpleRoundRobinArbiterC is then wired to Msp430Adc12P.  The Msp430Adc12P
component then uses this interface to perform run time checks to ensure that
only the client that currently has access to the ADC resource is able to
use it:</p>
<pre class="literal-block">
async command error_t Msp430Adc12SingleChannel.getSingleData[uint8_t id]() {
  if (call ADCArbiterInfo.clientId() == id){
    configureChannel()
    // Start getting data
  }
  else return ERESERVE;
}

async command error_t Msp430Adc12FastSingleChannel.configure[uint8_t id]() {
  if (call ADCArbiterInfo.clientId() == id){
    clientID = id
    configureChannel()
  }
  else return ERESERVE;
}

async command error_t Msp430Adc12FastSingleChannel.getSingleData[uint8_t id]()
{
  if (clientID = id)
    // Start getting data
  else return ERESERVE;
}
</pre>
<p>In order for these runtime checks to succeed, users of the
Msp430Adc12SingleChannel and Msp430Adc12FastSingleChannel interfaces will have
to match their client id's with the client id of a corresponding Resource
interface.  This can be done in the following way:</p>
<pre class="literal-block">
generic configuration Msp430Adc12ClientC() {
  provides interface Resource;
  provides interface Msp430Adc12SingleChannel;
}
implementation {
  components Msp430Adc12C;
  enum { ID = unique(MSP430ADC12_RESOURCE) };

  Resource = Msp430Adc12C.Resource[ID];
  Msp430Adc12SingleChannel = Msp430Adc12C.SingleChannel[ID];
}
</pre>
<pre class="literal-block">
generic configuration Msp430Adc12FastClientC() {
  provides interface Resource;
  provides interface Msp430Adc12FastSingleChannel;
}
implementation {
  components Msp430Adc12C;
  enum { ID = unique(MSP430ADC12_RESOURCE) };

  Resource = Msp430Adc12C.Resource[ID];
  Msp430Adc12FastSingleChannel = Msp430Adc12C.SingleChannel[ID];
}
</pre>
<p>Since these are generic components, clients simply need to instantiate them
in order to get access to a single Resource interface that is already
properly coupled with a Msp430Adc12SingleChannel or Msp430Adc12FastSingleChannel
interface.</p>
<p>Take a look in the tinyos-2.x source tree under tinyos-2.x/tos/chips/adc12
to see the full implementation of these components in their original context.</p>
</div>
<div class="section">
<h2><a id="id13" name="id13">ResourceRequested</a></h2>
<p>On the eyesIFXv2 platform, both the radio and the flash need access to the bus
provided by the Usart0 component on the Msp430 microcontroller.  Using
a simple cooperative arbitration policy, these two components should able to
share the Usart resource by only holding on to it as long as they need it and
then releasing it for use by the other component.  In the case of the MAC
implementation of the Tda5250 radio component, however, the Msp430 Usart
resource needs be held onto indefinitely.  It only ever considers releasing the
resource if a request from the flash component comes in through its
ResourceRequested interface.  If it cannot release it right away (i.e. it is in
the middle of receiving a packet), it defers the release until some later point
in time.  Once it is ready to release the resource, it releases it, but then
immediately requests it again.  The flash is then able to do what it wants with
the Usart, with the radio regaining control soon thereafter.</p>
<p>In the CsmaMacP implementation of the Tda5250 radio we see the ResourceRequested
event being implemented:</p>
<pre class="literal-block">
async event void ResourceRequested.requested() {
  atomic {
    /* This gives other devices the chance to get the Resource
       because RxMode implies a new arbitration round.  */
    if (macState == RX) call Tda5250Control.RxMode()();
  }
}
</pre>
<p>Through several levels of wiring, the Tda5250 interface is provided to this
component by the Tda5250RadioP component:</p>
<pre class="literal-block">
module Tda5250RadioP {
  provides interface Tda5250Control;
  uses interface Resource as UsartResource;
  ...
}
implementation {
  async command error_t Tda5250Control.RxMode() {
    if(radioBusy() == FALSE)
      call UsartResource.release();
      call UsartResource.request();
  }
  event void UsartResource.granted() {
    // Finish setting to RX Mode
  }
  ...
}
</pre>
<p>Although the full implementation of these components is not provided, the
functionality they exhibit should be clear.  The CsmaMacP component receives the
ResourceRequested.requested() event when the flash requests the use of the
Msp430 Usart0 resource.  If it is already in the receive state, it tries to
reset the receive state through a call to a lower level component.  This
component checks to see if the radio is in the middle of doing anything (i.e.
the radioBusy() == FALSE check), and if not, releases the resource and then
requests it again.</p>
<p>To see the full implementations of these components and their wirings to one
another, please refer to the tinyos-2.x source tree under
tinyos-2.x/tos/chips/tda5250, tinyos-2.x/tos/chips/tda5250/mac,
tinyos-2.x/tos/platforms/eyesIFX/chips/tda5250, and
tinyos-2.x/tos/platforms/eyesIFX/chips/msp430.</p>
</div>
<div class="section">
<h2><a id="resource-configure" name="resource-configure">Resource Configure</a></h2>
<p>The Msp430 Usart0 bus can operate in three modes: SPI, I2C,
and UART. Using all three concurrently is problematic: only one should
be enabled at any given time. However, different clients of the bus might
require the bus to be configured for different protocols. On Telos, for example
many of the available sensors use an I2C bus, while the radio and flash chip use
SPI.</p>
<p>A component providing the SPI service on top of the shared Usart component looks
like this:</p>
<pre class="literal-block">
generic configuration Msp430Spi0C() {
  provides interface Resource;
  provides interface SpiByte;
  provides interface SpiPacket;
  ...
}
implementation {
  enum { CLIENT_ID = unique( MSP430_SPIO_BUS ) };

  components Msp430SpiNoDma0P as SpiP;
  components new Msp430Usart0C() as UsartC;
  SpiP.ResourceConfigure[ CLIENT_ID ] &lt;- UsartC.ResourceConfigure;
  SpiP.UsartResource[ CLIENT_ID ] -&gt; UsartC.Resource;
  SpiP.UsartInterrupts -&gt; UsartC.HplMsp430UsartInterrupts;
  ...
}
</pre>
<p>And one providing the I2C service looks like this:</p>
<pre class="literal-block">
generic configuration Msp430I2CC() {
  provides interface Resource;
  provides interface I2CPacket&lt;TI2CBasicAddr&gt; as I2CBasicAddr;
   ...
}
implementation {
  enum { CLIENT_ID = unique( MSP430_I2CO_BUS ) };

   components Msp430I2C0P as I2CP;
   components new Msp430Usart0C() as UsartC;
   I2CP.ResourceConfigure[ CLIENT_ID ] &lt;- UsartC.ResourceConfigure;
   I2CP.UsartResource[ CLIENT_ID ] -&gt; UsartC.Resource;
   I2CP.I2CInterrupts -&gt; UsartC.HplMsp430UsartInterrupts;
   ...
}
</pre>
<p>The implementation of the ResourceConfigure interface is
provided by both the Msp430SpiNoDma0P and the Msp430I2C0P.  In the
two different components, the same Msp430Usart0C component is used,
but wired to the proper implementation of the ResourceConfigure
interface. In this way, different instances of the Msp430Usart0C
can each have different configurations associated with them, but
still provide the same functionality.</p>
<p>Take a look in the tinyos-2.x source tree under
tinyos-2.x/tos/chips/msp430/usart to see the full implementation of
these components along with the corresponding Uart implementation.</p>
</div>
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